Post on 23-Aug-2020
The “Top” Priority at the LHCTao Han
University of Wisconsin-Madison
(LHC Workshop, Princeton, March 21, 2007)
With Vernon Barger and Devin Walker hep-ph/0612016;
Rakhi Mahbubani, Devin Walker and Lian-Tao Wang, to appear.
The “Top” Priority at the LHC – p.1/40
The “Top” Priority at the LHC– Top quark: A Window to New Physics
Tao Han
University of Wisconsin-Madison
(LHC Workshop, Princeton, March 21, 2007)
With Vernon Barger and Devin Walker hep-ph/0612016;
Rakhi Mahbubani, Devin Walker and Lian-Tao Wang, to appear.
The “Top” Priority at the LHC – p.2/40
An exciting time:
The triple “Coincidence”
(1). A highly successful theory: the Standard Model.
T. Han, UW-Madison – p.3/40
An exciting time:
The triple “Coincidence”
(1). A highly successful theory: the Standard Model.
(2). Terascale new physics must exist!
T. Han, UW-Madison – p.4/40
An exciting time:
The triple “Coincidence”
(1). A highly successful theory: the Standard Model.
(2). Terascale new physics must exist!
(3). Upcoming the LHC: Uncover the underlying physics.
T. Han, UW-Madison – p.5/40
Terascale Physics at the LHC
Unitarity argument for WLWL scattering⇒ New physics must show up at the Terascale: A Higgs boson mH < 1 TeV or alike.
Naturalness argument for a mH or EW scale⇒ New physics needed beyond H0....
Gauge coupling unification⇒ New threshold at the Terascale.
Particle dark matter⇒ WIMP at the Terasacle natural.
T. Han, UW-Madison – p.6/40
Terascale Physics at the LHC
Unitarity argument for WLWL scattering⇒ New physics must show up at the Terascale: A Higgs boson mH < 1 TeV or alike.
Naturalness argument for a mH or EW scale⇒ New physics needed beyond H0....
Gauge coupling unification⇒ New threshold at the Terascale.
Particle dark matter⇒ WIMP at the Terasacle natural.
What exact form is it realized in nature?Fundamental scalar in a weakly coupled theory? (SUSY or alike)Composite Higgs and strongly interacting dynamics? (TC, Little Higgs)Low scale string/gravity? (Large extrad dim., RS, ...)
Dark matter connection? (WIMP: LSP ...)
The LHC will tell!T. Han, UW-Madison – p.7/40
In anticipation of the LHC
Re-discover the SM!
DY: Z → ℓ+ℓ−,W± → ℓ±ν
Jet inclusive ET (j)
Heavy quarks Υ, J/ψ, bb̄, tt̄
Gauge boson pairs WW,ZZ
Large E/T +X
T. Han, UW-Madison – p.8/40
Top quarks at the LHC
LHC is a top factory:
Event rate: 800K tt̄ / fb−1, or 8 Hz @1034/cm2/s !From Tevatron to LHC: tt̄ increased by 100; EW increased by 10.
T. Han, UW-Madison – p.9/40
Top quarks at the LHC:
Production well predicted in the SM:At the LHC: gg 90%, qq̄ 10%;
(At the Tevatron: gg 10%, qq̄ 90%.)
T. Han, UW-Madison – p.10/40
Why Top Quarks? bread & butter:
Top quark exists,as the heaviest particle in the SM.
mt is the most preciselymeasured quark mass —Important for precision physicsof the SM and beyond: 80.3
80.4
80.5
150 175 200
mH [GeV]114 300 1000
mt [GeV]
mW
[G
eV]
68% CL
∆α
LEP1 and SLD
LEP2 and Tevatron (prel.)
Top decays before it hadronizes:Good test ground for QCD, spin correlation, couplings, CP property...
Possible deep connection to electroweak symmetry-breaking:mt ≈ v/
√2.
T. Han, UW-Madison – p.11/40
Why Top Quarks? A window to new physics:
Largest Yukawa coupling (proportional to mt, cot β):H, A → tt̄.
Strong Dynamics (TC, Topcolor/Top See-Saw, Little Higgs):ρTC , ηTC , πTC , ZL → tt̄.
Extra-dimensions (warped and universal):gKK , GKK → tt̄.
Supersymmetry (t̃ often the lightest squark):t̃R → tχ̃0.
LH with T-parity (theories with naturalness argument):T → tA0.
... ...
T. Han, UW-Madison – p.12/40
The Remainder of the Talk
Search I: tt̄ Resonant Production
Backgrounds
Reconstruction Methods
Search for New Physics
Search II: tt̄+E/T Signal
Backgrounds
Signal Events Reconstruction
Conclusions
T. Han, UW-Madison – p.13/40
Search I: tt̄ Resonant Production
“Bump searches” in the Mtt̄ distribution.
Representative features:
Model Class Spin-0 Spin-1 Spin-2
Technicolor/Topcolor/RS × (nrw/brd) × (nrw/brd) × (narrow)
MSSM × (narrow)
Little Higgs × (narrow) × (narrow)
A model-independent approach:Parametrize each resonance with a few parameters:m, Γ, σ-normalization, chirality, CP violation
T. Han, UW-Madison – p.14/40
Strategy
To maximumly extract the resonant information:(Spin, chirality couplings, CP properties ...)
=⇒ Need full kinematics and top-ID.
Using the clean channel: “Semi-leptonic"tt̄ → bℓ±ν, bjj → 2b 2j ℓ± E/T .
Total Hadronic Channel: σt/tbar × (6/9)2 =⇒ large background, no top-ID ...
Semi-Leptonic Channel: σt/tbar × 6/9 × 2/9 × 2 =⇒ current interest.
Pure leptonic Channel: σt/tbar × (2/9)2 =⇒ small rate, incomplete kinematics ...
Semi-leptonic/hadronic ratio: 2/3
Leptonic/hadronic ratio: 1/9
We propose new/refined top reconstruction methods.
Take advantage of the tops being highly boosted.
T. Han, UW-Madison – p.15/40
Event Selection
When indicated we apply the following acceptance cuts:Jets: pT > 20 GeV, |ηj | ≤ 2.5
Leptons: pT ≥ 20 GeV, |ηl| ≤ 2.5, ∆R > 0.4
ET/ ≥ 20 GeV (tightened the ATLAS cuts)
Minimum Transverse Mass: MT (tt̄) > 600 GeV
Detector effects (Gaussian smearing) taken into account:
∆E
E=
a√
E/GeV⊕ b.
CMS ECAL resolution: a = 0.03, b = 0.005
CMS HCAL resolution: a = 1.1, b = 0.05
pT CMS resolution: a = 1.5 × 10−4, b = 0.05
ATLAS/CMS simulations:SM processes not a threat to the top-quark sample.
T. Han, UW-Madison – p.16/40
Kinematics Reconstruction
Present two schemes:Reconstruction of the missing neutrino!
(MW , mt)* scheme: the masses as known inputs.
Small angle scheme: t decay products collimated.
*Similar to the three-constraint kinematic fit used at the Tevatron to determine the top mass.CDF Collaboration, Phys. Rev. Lett. 80 (1998) 2767;D0 Collaboration, Phys. Rev. D27 (1983) 052001.
T. Han, UW-Madison – p.17/40
(MW , mt) Scheme
Step 1: Reconstruct W boson with the constraint: M2W = m2
lν .Yields two-fold ambiguity for the neutrino longitudinal momentum:
pνL =ApeL ± Ee
q
A2 − 4 ~p 2eT
~E/T2
2 p2eT
, where A = M2W + 2 ~peT · ~E/T .
• If A2 − 4 ~p 2eT
~E/T2≥ 0, then choose the value that reconstructs m2
t = m2lνb.
• If not, go to Step 2.
Expected distributions from this reconstruction alone:
T. Han, UW-Madison – p.18/40
(MW , mt) Scheme Cont’d
Step 2: If Step 1 gives complex solutions, first reconstruct top quark with: m2t = m2
lνb.
Yields two-fold ambiguity for the neutrino longitudinal momentum:
pνL =A′ pblL ±
q
p2blLA′ 2 + (E2
bl − p2blL) (A′ 2 − 4E2
blE/2T ) 2
2 (E2bl − p2blL)
,
A′ = m2t −M2
bl + 2 ~pblT · ~E/T .
• If p2blLA′ 2 + (E2
bl − p2blL) (A′ 2 − 4E2
blE/2T ) 2 ≥ 0, then choose M2
W = m2lν .
• If not, go to Step 3.
Expected distributions from this reconstruction alone:
T. Han, UW-Madison – p.19/40
(MW , mt) Scheme Cont’dStep 3: If the solution fails to reconstruct for both times, the event is discarded.• The discarded event rate ∼ 16%.• Choosing to keep the discarded solutions (by taking the real components)results in a distortion of high/low invariant mass tails.
Resulting reconstructions of mt and Mtt:
We have the full tt̄ kinematics!T. Han, UW-Madison – p.20/40
Small Angle Selection Scheme
Wish not to rely on mt input, because ...High tt̄ invariant mass limit: Tops quarks highly energetic/boosted:
Expect a small angle θlν , or in turn, θbW to give the correct solution.
Consider cos θbW :
MT > 0, 600, and 1000 GeV.
T. Han, UW-Madison – p.21/40
Small Angle Selection Scheme Cont’d
Thus,Again, M2
w = m2lν =⇒ two-fold ambiguity in pνL and thus pW .
The scheme: selecting the solution with the smaller angle between the b and the W .
Resulting reconstruction of mt and mtt:
Note the leakage at the high mtt region...
T. Han, UW-Madison – p.22/40
Search for New Resonances inmtt
Search for integer spin resonances via
gg → φ0 → t̄ + t
qq̄ → V1 → t̄ + t
qq̄, gg → h̃2 → t̄ + t
where φ, V and h̃ are J = 0, 1, 2 resonaces.
Parametrize each interaction with five parameters:m – mass of the resonance (1 TeV for benckmark study)
Γ – total width• Γφ = 0.5(mφ/TeV )3, ΓV = 5%mV , Γh̃ = 1.2%mh̃
ω2 – cross section normalization factor• ωφ = 1 recovers SM higgs• ωV = 1 recovers Z′ with electroweak couplings• ωh̃ = 1 recovers RS graviton
Chirality CP violation ...
T. Han, UW-Madison – p.23/40
Resonance Distributions inmtt
(MW , mt) and small angle selection, respectively:
Γtot = 2% m, 5% m, 20% m.
T. Han, UW-Madison – p.24/40
Discovery Potential
Determine minimal ω for a 5σ discovery
S/√
B + S = 5
T. Han, UW-Madison – p.25/40
Angular Distributions in tt̄ c.m. frame
(MW , mt) and small angle selection, respectively
Red dashed: scalar → flat;Black dots: Chiral vector → d111 ⇒ (1 + cos θ∗)2;Blue dash-dots: graviton from gg → d22±1 ⇒ sin2 θ∗;
Black solid: graviton from qq̄ → d21±1 ⇒ sin4 θ∗ + ....
T. Han, UW-Madison – p.26/40
Search II: tt̄ + E/T Signal
Quite generically,
pp → T T̄X → tt̄ A0A0X
→ tt̄ A0A0X → bj1j2 b̄ℓ−ν̄ A0A0 X + c.c.
600 800 1000 1200 1400m
T (GeV)
0.01
0.1
1
10
100
Prod
uctio
n cr
oss-
sect
ion
(pb)
QCD top production
Fermionic T production
Scalar T production
ttZ
T. Han, UW-Madison – p.27/40
Features: No Bump, but muchE/T
Due to more missing particles from both T and T̄ ,no pν can be reconstructed. Instead, may lead to larger E/T :
0 200 400 600 800 1000p
Tmiss (GeV)
1e-06
1e-05
0.0001
0.001
0.01
0.1
Dif
fere
ntia
l cro
ss-s
ectio
n (p
b G
eV-1
) QCD top production
mA
= 800 GeV
mA
= 200 GeV
ttZ production
mT
= 1 TeV.
T. Han, UW-Madison – p.28/40
It’s Good or Bad: A complex mrt
pν reconstruction by MW may yield complex solutions:
0 200 400 600 800Re(m_t)
0
100
200
300
400
500
Im(m
_t)
10^-10 < wgt < 10^-1210^-8 < wgt < 10^-1010^-6 < wgt < 10^-810^-4 < wgt < 10^-610^-2 < wgt < 10^-41 < wgt < 10^-2weight > 1
0 500 1000 1500 2000Re(m_t)
0
100
200
300
400
500
Im(m
_t)
10^-10 < wgt < 10^-1210^-8 < wgt < 10^-1010^-6 < wgt < 10^-810^-4 < wgt < 10^-6
T. Han, UW-Madison – p.29/40
Back-to-back t − t̄
SM t − t̄ are back-to-back in the transverse plane,while those from T T̄ are kicked randomly:
0 1 2 3Angle in transverse plane (rad)
0
0.5
1
1.5
2
2.5
3
Res
cale
d di
ffer
entia
l cro
ss-s
ectio
n (r
ad-1
)
QCD top production
mA
= 800 GeV mA
= 200 GeV
T. Han, UW-Madison – p.30/40
LHC Reach for T T̄ Signal
After judicious cuts, plus
|mt − mrt |2 > 110 GeV,
200 300 400 500 600 700 800m
A (GeV)
0
0.5
1
1.5
2
S / B
0
5
10
15
20
S / S
QR
T(B
) (f
or lu
min
osity
100
fb-1
)
S / B
S / SQRT(B)
mT
= 1 TeV.T. Han, UW-Madison – p.31/40
Conclusions
LHC is a top factory – providing 8 million tt̄’s per 10 fb−1 .Good channel to probe physics beyond SM.May serve as an early indicator for new physics.
For the resonant signal tt̄: Two methods to reconstructsemi-leptonic tt̄ events at high-invariant mass,to study resonant spin, Chiral coupings, CP properties ...
For non-resonant signal tt̄ + E/T : Observation of thesemi-leptonic channel promising, but kinematics difficult:No information for the missing particle mass mA, mχ0.
T. Han, UW-Madison – p.32/40
Conclusions
LHC is a top factory – providing 8 million tt̄’s per 10 fb−1 .Good channel to probe physics beyond SM.May serve as an early indicator for new physics.
For the resonant signal tt̄: Two methods to reconstructsemi-leptonic tt̄ events at high-invariant mass,to study resonant spin, Chiral coupings, CP properties ...
For non-resonant signal tt̄ + E/T : Observation of thesemi-leptonic channel promising, but kinematics difficult:No information for the missing particle mass mA, mχ0.
Top quark studies are of high priority!
T. Han, UW-Madison – p.33/40
Background Considerations
W + jets, Z + jets, WW, WZ, ZZ backgrounds:Consider table of efficiencies reproduced from the ATLAS TDR (Volume II, p. 624).The expected events are in the last column.
Process Efficiency with As before, As before, Events
plT > 20 GeV with with per 10 fb−1
ET miss > 20 GeV plus Njet ≥ 4 plus Nb−jet ≥ 2
cuts cut cut
tt̄ signal 64.7 21.2 5.0 126,000
W+ jets 47.9 0.1 0.002 1658
Z+ jets 15.0 0.05 0.002 232
WW 53.6 0.5 0.006 10
WZ 53.8 0.5 0.02 8
ZZ 2.8 0.04 0.008 14
Total Background 1922
S/B 65
T. Han, UW-Madison – p.34/40
Background Cont’d
W + jets, W + 4 jets, Wbb + 2j, Wbb + 3j backgrounds:Efficiencies from CMS TDR (Volume II, p. 238).
Semi- Other
leptonic tt̄ tt̄ W+4j Wbb+2j Wbb+3j S/B
Before cuts 365k 1962k 82.5k 109.5k 22.5k 5.9
L1+HLT Trigger 62.2% 5.3% 24.1% 8.35% 8.29% 7.8
Four jets ET > 30 GeV 25.4% 1.01% 4.1% 1.48% 3.37% 9.9
plepton
T cut 24.8% 0.97% 3.9% 1.41% 3.14% 10.3
b-tag criteria 6.5% 0.24% 0.064% 0.52% 0.79% 25.4
Kinematic fit 6.3% 0.23% 0.059% 0.48% 0.72% 26.7
Cross section (pb) 5.21 1.10 0.10 0.08 0.05 26.7
Scaled to L = 1 fb−1 5211 1084 104 82 50 26.7
T. Han, UW-Madison – p.35/40
Background Cont’d
Reconstructing the hadronic decay (t̄, say):From ATLAS collaboration (ATLAS TDR,Volume II, p. 625): The t̄ is reconstructed viathe hadronic decay b̄jj. The wrong b may have some contamination (shaded area).
0
2000
4000
0 100 200 300 400mjjb (GeV)
σ = 11.9 GeVE
vent
s/4
GeV
T. Han, UW-Madison – p.36/40
Background Cont’d
At high Mtt̄, the tops are boosted.That helps select (ℓ+ b), rather than (ℓ+ b̄).
T. Han, UW-Madison – p.37/40
Background Cont’d
Lepton isolation does not effect the signal appreciably:∆R > 0.4
T. Han, UW-Madison – p.38/40
Small Angle Selection Scheme Cont’d
High Invariant mass tail is due to smearing and the wrongsolution in small angle selection.
A solution: Provide an incrementally higher transverse masscut depending on expected resonance.
MT >650 and 950 GeV.T. Han, UW-Madison – p.39/40
Chirality
Forward/Backward Asymmetry for Parity-Violation:
Ahadf =
NF − NB
NF + NB
NF (NB) is the number of events with the top quark momentum ~ptop in the forward(backward) direction defined relative to the quark moving direction ~pq ,
“Forward" for the final state top is thus defined relative to the boost direction from theresonance c.m. frame (because the valence quarks tend to carry a higher-momentumfractions than the sea (anti-) quarks.)
Gluon contributions are homogeneous and subtracted out.
Similarly, one may consider CP-asymmetry.(work in progress.)
T. Han, UW-Madison – p.40/40